Hydraulic cylinders have been used for the actuation of valves for many years. However, advances in microprocessor based control systems now require an accurate and dependable feedback signal to determine valve position during operation.
This need for accuracy in positioning is especially acute in fluidized catatylic cracking units (FCCU) used in the refining of petroleum products. The plug valves therein utilized require actuator control of the position of the plug valve to very high degrees of accuracy since very small movements of the conically shaped plug valve can result in non-linear proportional changes in the flow area, and resultant mass flow, by several percentage points. In the case of the typical older FCCU plug valves actuators which operate ,with the regenerated catalyst plug valves and to therefore control the temperature of the riser, a movement of less that 0.030 inch will change the temperature of the riser 1 degree Fahrenheit. This critical movement therefore affects the flow performance, the yield, and the stability of the FCCU.
The method of producing accurate position control has in the past been accomplished through the use of linear variable displacement transducers (LVDT) mounted by various manufacturers in the actuator and within the cylinder. The more modern method of measuring valve position is to indirectly measure piston position through a linear displacement transducer (LDT) using electronic components mounted within the cylinder. The LDT measures the actual piston position inside the cylinder and a signal is sent to the position control amplifier corresponding to that particular piston position. These devices fit in the rear of the cylinder directly. The position control amplifier compares the feedback position to that of a command signal from the process distributive control system. From this comparison a signal is produced to correct any error that exists in position as a difference between the two signals. This correction error signal is sent to the servo valve which converts the electronic signal into hydraulic fluid movement directed to the hydraulic cylinder. This error signal can also be used for alarming a control room if nominal operating parameters are exceeded.
However, such systems suffered from a major drawback. If for any reason the LDT should fail, its replacement required the entire hydraulic cylinder to first be removed from service. In the context of a FCCU, this is a dangerous and expensive operation. Moreover, to remove the LDT of previous systems from the cylinder, the cylinder must first be depressurized. Any remaining fluid would flow or leak out. If this operation is done in the field, as is often the case, the leaking of hydraulic fluid creates an extremely hazardous event. With a flash point of 325.degree. F. and the surrounding processing lines having skin temperatures sometimes exceeding 600.degree. F., the risk of fire (and potential explosion of the entire FCCU) is not trivial. Also, the hydraulic fluid, at elevated temperatures given this particular environment, creates a direct hazard for repair crews. Moreover, the leakage of hydraulic fluid, even in relatively small amounts (i.e., one gallon) is now an event which must be reported to the United States Environmental Protection Agency. Thus, solutions were sought to end the dangers caused by leakage arising from the replacement of LDT units.